1. Field of the Invention
The present invention relates to an image forming apparatus which utilizes an electrophotographic method, electrostatic recording method or the like to form a recorded image by developing an electrostatic image formed on an image bearing member using developer, and particularly can be advantageously applied to an image forming apparatus that uses two-component developer comprising a toner and a carrier as a developer.
2. Related Background Art
An image forming apparatus that employs an electrophotographic method is known in which, for example, a latent image that was formed on an image bearing member thereof is developed using developer and visualized as a toner image. In a developing device that uses two-component developer comprising a toner and a carrier, it is important that the mixing ratio for the toner (T) and the carrier (C) of the developer, that is, T/D (D=T+C) (indicates the “toner density” in the developer; hereunder also referred to as “T/D ratio”) is kept constant. Therefore, several methods including a light reflection detection method, an inductance method and a patch detection method have already been proposed for an auto toner replenisher (ATR) as means of detecting the toner density of a developer in the developing device and controlling the toner density.
However, the following problem exists with regard to the light reflection detection method. That is, when the toner of the developer uses as a colorant a coloring agent, such as carbon, that exhibits the same optical properties as the carrier, it is not possible to detect the T/D ratio on the basis of the difference in the light reflection intensity of the developer.
Further, the following problem exists with regard to the inductance method that performs detection based on changes in the magnetic permeability of the developer. That is, the charging charge amount of the toner may be increased and the fluidity improved for the purpose of enhancing image quality. In this case, even if the developer is agitated and the like, a change in the density of the carrier may occur due to repulsion of the charged developer. As a result, since the magnetic permeability of the developer changes even though the T/D ratio is the same, there are cases in which an accurate T/D ratio is not detected.
Thus, the following method is known as a method that measures the T/D ratio of developer in an auto toner replenisher (ATR). That is, a latent image of a constant potential (reference patch latent image) is formed on an image bearing member separately to the normal image formation, and a patch image (image for image density control) is then formed as a reference density pattern by directly developing this latent image. Subsequently, the density of this patch image on an image bearing member or on a transferring member after the patch image was transferred to the transferring member from an image bearing member is optically detected using an image density sensor as image density detection means. The T/D ratio is then determined utilizing the fact that the density of the patch image correlates with the T/D ratio. This method is called “patch detection method” (patch detection ATR).
In many conventional image forming apparatuses an image density sensor for detecting the density of a patch image is provided facing an image bearing member or a member for transfer (recording material carrying member, intermediate transferring member or the like) to which a patch image is transferred from an image bearing member, in a position in the vicinity of the center in the thrust direction (direction roughly perpendicular to the direction of movement of the surface of the image bearing member or the member for transfer).
A conventional developing device will now be described referring to FIG. 2. In the figure, a developing device 1 comprises developer conveying means 106 and 107 inside a developing container 101. Developer that was supplied by the developer conveying means 106 and 107 to the surface of a developing sleeve 102 as a developer carrying member is carried by rotation of the developing sleeve 102 to a cylindrical electrophotographic photosensitive member (photosensitive member) as an image bearing member, that is, to a portion (developing area) facing a photosensitive drum 3.
Further, in the example in FIG. 2, the inside of the developing container 101 is divided into a developing chamber (first chamber) 110 and an agitating chamber (second chamber) 111 by a partition wall 109 that extends in a vertical direction. A two-component developer comprising non-magnetic toner and a magnetic carrier is contained inside the developing chamber 110 and agitating chamber 111.
In the developing chamber 110 and the agitating chamber 111 are respectively disposed a first developer conveying means (first screw) 106 and a second developer conveying means (second screw) 107 that are generally of a screw type. The first screw 106 agitates and carries the developer in the developing chamber 110. When toner is supplied from a toner replenishing container (not shown in the figure) through a toner replenishing port (not shown in the figure), the second screw 107, under the control of an auto toner replenisher (ATR), carries the supplied toner and agitates it with the developer that is already in the agitating chamber 111 to make the toner density uniform. The toner replenishing port is normally provided near the upstream side in the direction in which developer is carried by the second screw 107 inside the agitating chamber 111.
The first screw 106 and the second screw 107 carry the developer in respectively opposite directions. That is, the first screw 106 carries developer from the rear side of the page space towards the front side in FIG. 2, and the second screw 107 carries developer from the front side of the page space towards the rear side in FIG. 2. Further, in the partition wall 109, at the edge between the front side and the rear side in FIG. 2, developer passages (not shown in the figure) are formed that interconnect the developing chamber 110 and the agitating chamber 111. Accordingly, through the carrying forces of the first screw 106 and the second screw 107, developer in the developing chamber 110 in which toner density decreased due to consumption of toner in the developing process moves into the agitating chamber 111 through one of the developer passages. Further, developer inside the agitating chamber 111 into which toner was replenished and agitated moves into the developing chamber 110 through the other developer passage.
During this developer circulation, conventionally a patch image is generally formed near the center in the thrust direction of the developing sleeve 102 (direction roughly perpendicular to the direction of movement of the surface), and the patch image is detected with an image density sensor 41 that is provided in a corresponding position.
However, the following problem was found to exist when toner replenishment is performed, as described above, by an auto toner replenisher (ATR) in accordance with a detection result obtained by detecting the density of a patch image formed in a center portion in the thrust direction of the developing sleeve 102. That is, when a patch image is formed at a center portion in the thrust direction of the developing sleeve 102 as described above, during the period that developer that was in a position inside the developing container 101 corresponding to the formation position of the patch image on the developing sleeve 102 when the patch image was formed circulates as far as the position of the toner replenishing port 108, toner in the developer is consumed by an image formed immediately after formation of the patch image. Therefore, in the case of replenishing toner based on a result obtained by detecting the density of the patch image, the T/D ratio when actually replenishing the toner may be different from that at the time of patch image formation, and therefore it is not possible to control the T/D ratio appropriately.
For example, in a case in which a high density image was formed immediately after patch image formation, the T/D ratio at the toner replenishing port will be in a lower state than the state of the developer at the time the patch image was formed. As a result, since the toner replenishment amount is insufficient, an appropriate T/D ratio will not be obtained.
The present invention was made in light of the above-described problems.
In this connection, Japanese Patent Application Laid-Open No. H8-220870 considered the problem that, in the case of copy modes in which there are large variations in the toner consumption amount, for example, when switching from continuous copying of black solid copies to continuous copying of blank copies, although toner replenishment is at a maximum during the continuous copying of black solid copies, when the copying of blank copies subsequently starts the balance between the toner replenishment amount and the toner density collapses, and it proposed providing image density detection means (means employing a light reflection detection method that detects the density of a patch image formed on a photosensitive drum) or developer density detection means (means employing a light reflection detection method that directly detects the toner density of the developer) at the rear side and front side in the longitudinal direction of the developing device, respectively. The invention according to Japanese Patent Application Laid-Open No. H8-220870 is directed at conducting toner replenishment control in accordance with the difference in the detection results of the two detection means to prevent the problem described in the foregoing discussion and also enable the size of the developing device to be reduced. However, the invention of Japanese Patent Application Laid-Open No. H8-220870 does not consider at all the problem of errors in toner replenishment control due to consumption of toner in an area past a patch image formation position in the developer carrying direction. Further, it contains no disclosure or suggestion with respect to converting a toner amount that is actually used in an area past a patch image formation position into a value indicating a fluctuation in the T/D ratio inside the developing device.